Phenol is a valuable commodity intermediate chemical which is among the top 10 organic chemical monomers produced in the US. Principal applications for phenol are as an intermediate to bisphenol A (used in turn to make polycarbonates); as a component in phenol-aldehyde resins, coatings, and adhesives; as a precursor to caprolactam (precursor to Nylon-6), detergents, antioxidants, and to a number of other chemicals which are used in diverse applications. Other hydroxylated aromatics are also of commercial importance. For example, cresols are used largely to manufacture herbicides and insecticides and antioxidants; 2,6-xylenol is the starting material for polyphenylene oxide, a thermoplastic with high heat and chemical resistance and excellent electrical properties developed by General Electric Co. Salicylic acid, dihydroxybenzenes including resorcinol, pyrocatechol, and hydroquinone are also derivatives.
Starting from benzene, the dominant current route to phenol is the "cumene peroxidation process" which requires multiple steps and produces coproduct acetone, markets for which are expected to grow much more slowly than those for phenol. This route also consumes propylene for which there are alternative applications which often produce greater economic return. In this process, benzene is first alkylated to cumene with propylene. Then in a second step, cumene is oxidized with air to the hydroperoxide which, in turn, is subsequently decomposed in the presence of acid to a 1:1 mixture of acetone and phenol: ##STR1##
Another route to phenol which accounts for less than 2% of industrial capacity in the US, is the "toluene oxidation route". In this route, toluene, not benzene, is catalytically oxidized to benzoic acid. In a second step, the benzoic acid is catalytically oxidatively decarboxylated to a 1:1 ratio of phenol and carbon dioxide, the latter being another usually undesired "greenhouse gas" byproduct. In addition to producing a low value byproduct, this route is also capital intensive due to the need for a complex product and catalyst recovery scheme: ##STR2##
Still other obsolete routes to phenol through halobenzene intermediates are known in the art, but these are no longer practiced commercially. Cresols and xylenols can be prepared by methylation of phenol with methanol in gas or liquid phase processes.
A commercially viable process for the direct, one-step oxidation of benzene to phenol (equation 3) would not only be simpler than dominant routes now practiced, but would also enable phenol or its derivatives to be marketed unencumbered by the need to find outlets for acetone or carbon dioxide coproducts and, furthermore, would consume no propene. ##STR3##
A number of workers have produced phenol from benzene over the years using molecular oxygen (or air) as the oxidant over a variety of catalysts, usually at high temperature. Unfortunately, ineluctable deep oxidations of benzene generally occur at the needed temperatures which lead to ring cleavage products such as carbon dioxide, carboxylic acids or anhydrides such as maleic anhydride, in toto resulting in poor selectivity from hydroxylation.
More selective hydroxylation of benzene without ring cleavage can be achieved at reasonable space-time yields using other oxidants such as hydrogen peroxide, nitrous oxide, tert-butyl or cumyl hydroperoxide, but these oxidants are up to 50 times more costly per oxygen equivalent than is dioxygen. In still other catalytic processes, molecular oxygen serves as the oxidant, but a stoichiometric co-reactant such as carbon monoxide or hydrogen must be co-fed to the catalyst. This practice not only augments the process costs to prohibitive levels, but also represents an engineering challenge to overcome heightened possibilities for uncontrolled oxidations or explosions. Certain of these processes are patented or described in publications; see, for example, K.-H. Chao et al., U.S. Pat. Nos. 4,982,015 and 4,992,600; Nemeth et al., U.S. Pat. No. 5,233,097; Kharitinov et al., U.S. Pat. No. 5,110,995; Sasaki et al., Bull. Chem. Soc. Jpn., 2850 (1994); Jintoku et al., Chem.Lett. 1687 (1990); Miyake et al., Appl. Cat. A, 131, 33 (1995); A. Matsudo et al., Jap. Pat. No. J03236338-A (1991).
In contrast, there is no previously known route to produce phenol from benzene which uses molecular oxygen as the sole oxidant with no requirement for a coreductant that yields phenol with sufficient selectivity and at sufficient space-time-yield so as to be commercially viable. The present invention provides such a process using O.sub.2 with low severity reaction conditions and good selectivity. An advantage of the present invention is to provide a method for the hydroxylation of aromatic compounds which does not require the added reagent and engineering costs and operational risks associated with the use of coreductants such as hydrogen or carbon monoxide. A further advantage of the present invention is to provide a method for the hydroxylation of aromatics which can use molecular oxygen as the terminal oxidant thereby avoiding the need for more expensive oxidants.